FIG. 1 shows a conventional magamp regulator circuit 10 having a transformer secondary winding 12 forming part of a transformer whose primary winding is not shown for purposes of simplicity. The magamp 10 delivers a pulse-width modulated output signal through a low pass filter consisting of L2 and C1 to output load R5. The output level of the signal is maintained at a substantially constant level by controlling the "duty cycle" (i.e., the time interval of each pulse) through feedback control means including feedback amplifier 14 which monitors the output level of the signal across load R5, controlling the relative conduction and nonconduction of a transistor Q1 forming part of a control circuit, the feedback signal being applied to the base electrode thereof.
The feedback amplifier constantly monitors the output voltage and adjusts the bias on Q1's base electrode such that the output voltage R5 is maintained constant. If the output voltage rises above the correct value, the feedback circuit will increase the current through Q1, which will reset L1 to a greater extent, thus decreasing the magamp duty cycle and lowering the output voltage applied to R5. If the output voltage decreases below the correct value, the feedback circuit will decrease the current through Q1, thereby allowing the magamp duty cycle to increase and raising the output voltage applied to R5. Control is maintained on a cycle by cycle basis.
Diode D1 prevents reverse current flow toward the magnetic amplifier. Diode D2 provides a current path for filter inductor L2 and output load R5 when D1 and magamp L1 are not conducting.
The conventional magamp control circuit of FIG. 1 has a number of problems, some of the significant ones being:
In the event of a short circuit condition across the load, the magamp circuit of FIG. 1 lacks the capability of restricting the output to load R5 due to a lack of sufficient internal bias provided to the transistor Q1 of the control circuit. For example, assuming a short circuit condition across load R5, the feedback amplifier circuit 12 holds the base of transistor Q1 at ground potential. In order for the control circuit to be functional, the emitter of Q1 must be maintained a 0.60 volts above ground potential. With the output at 0.60 volts, an uncontrolled amount of current can flow through the output. Previous circuits capable of short circuit current limiting required either an external bias or a far more elaborate circuit implementation;
The conventional magamp regulator, due to inherent circuit architecture responds too slowly to dynamic loading which results in undesirable fluctuations in output voltage;
The present magamp topology provides no means for compensating for an unwanted voltage reset applied to the saturable reactor as a result of power diode turn-off reverse recovery energy or reverse current leakage across protective diode D1. The best diodes presently available have a reverse recovery time that extends from 25 nsecs under ideal conditions to 200 nsecs and beyond. High temperatures exacerbate the problem, making it even more cumbersome to resolve. During the reverse recovery time of rectifier diode D1, the summation of the reverse transformer voltage plus the output voltage is applied to the magamp (inductor L1). This applied voltage via the effective shorting of diode D1 (reverse recovery time) results in an unwanted volt-second reset of the saturable reactor that limits the maximum available duty cycle of the voltage control circuit. A reduction in the duty cycle causes the output to drop in voltage which results in deregulation of the output. In circuits that combine high voltage with high current outputs, the parasitic reset may be sufficient to prevent the circuit from operating normally.